In an internal combustion engine (e.g., a diesel engine or a gasoline engine), exhaust gas therefrom may contain particulates such as soot. Such exhaust gas containing particulates is cleaned through collection of particulates by a filter. Also, when necessary, the filter is heated to a high temperature so as to remove, through burning, particulates accumulated on the filter. However, in the event of breakage of the filter or a like problem, unclean exhaust gas is directly emitted downstream of the filter. Thus, demand has been rising for a particulate sensor capable of detecting the amount of particulates contained in exhaust gas in order to directly measure the amount of particulates contained in exhaust gas and to detect malfunction of the filter.
Such a particulate sensor includes, for example, an inner metallic member having a gas introduction pipe, an outer metallic member, and an insulating spacer. The inner metallic member is maintained at a first potential different from a ground potential and is adapted to introduce exhaust gas into the gas introduction pipe. The outer metallic member is a tubular member which surrounds the radially outer circumference of the inner metallic member and which is attached to an exhaust pipe to thereby be maintained at the ground potential. The insulating spacer is a tubular member which is interposed between the inner metallic member and the outer metallic member so as to electrically insulate them from each other. A portion (gas contact portion) of the insulating spacer is exposed to the interior of the exhaust pipe and is to come into contact with exhaust gas flowing through the exhaust pipe. Such a particulate sensor is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2014-10099.